Orthogonal Frequency Division Multiplexing (OFDM) is the core physical layer technology in 4G Long Term Evolution (LTE) systems where a cyclic prefix (CP) is used to combat the inter-symbol interference (ISI) caused by the channel delays.
OFDM systems divide the available frequency spectrum into several sub-carriers. In order to obtain high spectrum efficiency, the frequency responses of the subcarriers in OFDM overlap and are orthogonal, which can be maintained with only a small loss in the signal to noise ratio (i.e. small gain in ISI) by using the CP.
In order to reduce the ISI completely, the length of the CP has to be longer than the maximum channel delay. This, of course, increases the system's overhead and decreases the OFDM system's spectrum efficiency (SE). Furthermore, it is expected that in the 5G system, the bandwidth of PFDM will be expanded to 100 or possibly even 200 megahertz (Mhz) and the corresponding OFDM symbol duration will be reduced to one-fifth or one-tenth of that employed in the 4G LTE system. As a result, the overheard that the CP occupies will be a higher ratio in one OFDM symbol, and the system's SE will significantly degrade as the ratio increases.
In practice, a moderate CP is adopted in the OFDM system in order to avoid too large an overhead of the CP. In this case, there may be excessive channel delays which are larger than the CP. Since these excessive channel delays have a delay spread longer than the CP, the CP cannot combat them, resulting in ISI which degrades the system performance.
Therefore, it is desirable to find a method and a communication device which is configured to allow the OFDM system to employ a moderate CP while reducing the impact of ISI due to excessive channel delays.